1. Field of the Invention
The present invention relates to the field of signal processing circuitry for use in bar code scanners, and more particularly, to signal processing circuitry for bar code scanners which is configured to be capable of changing various processing parameters on successive scans of a bar code symbol in order to achieve a better read rate compared with conventional approaches.
2. Background
Bar code scanners can either be categorized as fixed scanners or hand-held scanners. A fixed scanner is characterized by the fact that its location is fixed, so that the article being scanned must be brought to the scanner, while a hand-held scanner is characterized by the fact that it is mobile, so that the scanner can be brought to the article being scanned.
A bar code scanner, whether hand-held or fixed, operates by scanning a spot of light across the alternating black and white bars of a bar code symbol, detecting the light reflected off the symbol to form an analog signal having a magnitude proportional to the intensity of the reflected light, digitizing the analog signal to form a digital signal, where the width of the pulses making up the digital signal corresponds to the width of the bars in the bar code symbol, and then decoding the digital signal to determine the information which the symbol represents. The process of digitizing the analog signal is known as signal processing.
Various methods for signal processing have been employed in bar code scanners in the past, although each has been fraught with one or more problems when so employed.
One such method which is the "delay and compare" method, described more fully in U.S. Pat. No. 4,360,798, which is hereby fully incorporated by reference herein as though set forth in full.
According to the "delay and compare" method, one or more threshold signals are produced by delaying the analog signal and then offsetting the peaks and valleys of the delayed signal by a fixed amount, i.e., typically by one or more diode voltage drops. The threshold signals are then compared with the analog signal, and a digital signal is produced by starting or stopping the generation of digital pulses at the points where the analog signal intersects the threshold signals.
A first problem with the "delay and compare" method is that it will not successfully digitize weak analog signals, i.e., those that do not swing wide enough to cross the threshold signals. Nor is it very successful in digitizing analog signals produced from a label having a low contrast ratio, or analog signals produced when the scanner is positioned far from the article being scanned.
A second problem with this method is that it will not successfully digitize the analog signal when the scanner is positioned at or near the dead angle, i.e., at or near the angle at which the scanner intercepts a maximum of the reflected light from the symbol. This is because in this circumstance, the light detection circuitry typically becomes saturated, causing the label contents of the analog signal to become greatly reduced. Consequently, the threshold signals, which are determined at least partly by the peaks and valleys of the analog signal, will be distorted, and the resulting digital signal will be likewise distorted.
A third problem with this method is that the white to black and black to white transitions can be greatly distorted by incorrect threshold set-up due to circuit delay or different peak levels of the signal. Thus, for all the foregoing reasons, the "delay and compare" method has not proven to be optimal for use in bar code scanners.
A second signal processing method which has sometimes been employed in bar code scanners is the "threshold and compare" method. According to this method, one or more threshold signals are first formed by averaging the analog signal, such as by passing it through a filter or peak detector. As with the "delay and compare" approach, the threshold signals are then compared with the analog signal, and a digital signal is produced by starting and stopping the generation of digital pulses at the points where the threshold signals intersect the analog signal.
A first problem with this method is that it has little or no inherent noise immunity, in that any noise spikes on the analog signal that happen to cross the threshold signal will erroneously trigger the starting or stopping of a digital pulse.
A second problem with the "threshold and compare" method is that it will not successfully digitize high frequency analog signals, i.e., those produced by narrow bars, or those produced by rapidly scanning the light across the bars. This is because for high frequency signals, the method will not produce threshold signals fast enough which accurately represent the average of the analog signal at a particular point in time.
A third problem with this method is that, as with the "delay and compare" approach, the transitions can be distorted due to incorrect threshold set-up as a result of circuit delays or different peak levels of the signal. Thus, this method has not proven optimal either for use in bar code scanning.
A third method which has sometimes been employed in bar code scanners is a second derivative-based method described more fully in U.S. Pat. No. 4,000,397, which is hereby fully incorporated by reference herein as though set forth in full.
According to this approach, a signal representing the second derivative of the analog signal is formed, and a digital signal is then formed by starting and stopping the generation of digital pulses at the zero crossings of the second derivative signal. To minimize the erroneous detection of a zero crossing, the second derivative signal is only evaluated for the presence of zero crossings during selected gating periods. By using this approach, thresholding becomes less critical because the signal levels are produced from the transition of black to white or white to black bars.
A first problem with this method is that the circuitry required to implement it has heretofore been very bulky and complex, making it difficult to use in hand-held bar code scanners. There are numerous reasons for this, including the requirement that both 5V and 12V power supplies be used, the high bandwidth requirements of fixed scanners, and the corresponding use of bulky components such as inductors to form the high-Q filters needed to achieve high bandwidth, and the sheer complexity of the circuitry needed to define the gating periods.
For example, in the circuit described in U.S. Pat. No. 4,000,397, the gating periods are defined by starting and stopping the generation of gating pulses at the points where the first derivative of the analog signal crosses a threshold signal. Complex circuitry is required both to form the first derivative signal and the threshold signal.
A second problem with this approach is that the threshold signal used to form the gating pulses is not fixed, but, in a process known as active thresholding, is typically formed by peak-detecting the input signal. Consequently, this method will typically be susceptible to noise spikes for the same reasons discussed above with respect to the "threshold and compare" approach. For all these reasons, this method has not proven optimal either particularly for use in hand-held bar code scanners.
Accordingly, while those in the art have long recognized a need for a signal processing apparatus and method which has better properties compared to prior art methods with respect to compactness, noise immunity, depth of field, handling of weak signals, handling of signals at the dead angle, rapid formation of threshold signals, reduced distortion, or the like, no such apparatus and method has heretofore become available. Accordingly, an object of the present invention is to provide a signal processing apparatus and method which provides better properties along these lines.
Moreover, while those in the art have long recognized a need for a compact and cost effective derivative-based signal processing apparatus and method for use in hand-held bar code scanners, to date, no such apparatus and method has become available. It is believed that the complexity and size of the circuitry heretofore required to implement this method has deterred its use in hand-held bar code scanners in favor of the "delay and compare" and "threshold and compare" methods. Consequently, another object is to provide a derivative-based signal processing apparatus and method which is more suitable for being used in a hand-held bar code scanner.
Additional objects and advantages of the invention will be set forth in the description which follows or may be learned by practice of the invention.